Astronomical Foundations of the Astrological Houses

by Christopher A. Weidner

In order to understand the origins of the different house systems
and the problems connected with them, it is helpful to discuss
a few terms of celestial mechanics which are important from an
astrological perspective for the construction of the respective
systems.

The horizon system

The horizon is the most important reference level in astrology
when it comes to the houses because it forms the platform on which
we experience ourselves as observers of all phenomena. The horizon
divides the celestial sphere - at the centre of which the observer
is located - into a visible hemisphere above and an invisible hemisphere
below.

From an astronomical point of view, the matter is not that simple:
astronomers differentiate between the

true horizon and the

apparent horizon

The apparent horizon exactly corresponds to the horizon which
we perceive when we talk about the 'horizon'. We could also say:
the plane of the apparent horizon is on the same level as the
eyes of the observer.

The plane of the true horizon, however, is on the level of the
geocentre. Again, we could say: When we consider the true horizon,
we act
as if the observer was standing at the geocentre. For calculation
purposes, astrology uses the true horizon.

The difference between these two planes in reference to very distant
celestial bodies is extremely small and therefore basically negligible.

The horizon is the plane the observer perceives when looking at
celestial phenomena. We distinguish six reference points which
give us orientation on this plane:

East, South, West and North - give us orientation on the horizontal
plane, and

Zenith and Nadir - give us orientation on the vertical line.

Illustration 1: The horizon system with its three great circles.

The great circles in the horizon system

These points can be connected to form the so-called "great
circles".
They are circles in the celestial sphere whose centres align
with the centre of the celestial sphere itself. Three of these
great circles are especially important in astrology, because they
make up the framework of the celestial sphere from the viewpoint
of the observer (cf. illustration 1).

The horizon itself is one of the great circles: it connects
the East, South, West and North points, and thus defines the
horizontal axis (Ascendant and Descendant) of the horoscope.

The meridian is the great circle which is drawn around the
celestial sphere from the North point via Zenith, South point
and Nadir. It divides the celestial sphere into an Eastern and
a Western hemisphere, and forms the foundation of the meridian
axis in the horoscope, i.e. Medium Coeli and Imum Coeli.

The vertical great circle results from the connection of
East point, Zenith, West point and Nadir. It is not usually represented
in the horoscope, but is used to calculate some house systems.
Sometimes the East point is shown in the horoscope as "Anti-Vertex"
and the West point as "Vertex".

This framework of three great circles completely describes the
position of the observer in relation to all directions. But astrology
does not show the positions of the planets in direct reference
to the great
circles but as projections onto the ecliptic, the apparent orbit
of the Sun around the Earth, better known as Zodiac with its twelve
Zodiac sections or signs.

The ecliptic is another great circle in the celestial sphere,
which - depending on the position of the observer on
the Earth - has a different "obliquity" in relation
to the horizontal plane, and which intersects the three great
circles of the horizon system. These intersections are especially
important in astrology
because they consitute the cardinal points of the house system:

The intersections of the ecliptic and the horizon constitute
the Ascendant and Descendant.

The intersections of the ecliptic and the meridian make up
the Medium Coeli und Imum Coeli.

The intersections of the ecliptic and the vertical great circle
constitute two other points which are sometimes seen in the horoscope:
Vertex, the West point, and Anti-Vertex, the East point. Especially
the Vertex is often considered a sensitive point which is supposed
to be related to relationships and partner issues. Generally,
this point is - maybe wrongly - neglected in astrology.

The horizon in the horoscope

In the horoscope the horizon is represented by the Ascendant/Descendant
axis. The horizontal line divides the celestial sphere,
in the centre of which we experience ourselves, into a visible
and an invisible
area. We can observe this fact in the horoscope: all celestial
bodies below the Ascendant/Descendant axis
(1st and 2nd quadrant) were not visible at the time of birth, and
all bodies above this axis (3rd and 4th quadrant) were
visible (cf. illustration 2).

Illustration 2: The quadrants of the horoscope between the four
cardinal points of the house system

The meridian in the horoscope

The meridian is represented in the horoscope by the axis of
Imum Coeli and Medium Coeli (Midheaven) and divides the horoscope
wheel into an Eastern (1st and 4th quadrant) and a Western hemisphere
(2nd and 3rd quadrant). All celestial bodies in the Eastern hemisphere
are ascending, and all celestial bodies in the Western hemisphere
are descending (cf. illustration 2). The culmination is marked
by the Medium Coeli - the Midheaven: this is where, for example,
the Sun is at midday. This point is also called upper or midday
culmination because it is above the horizon. The opposite point
is symbolised by the Imum Coeli - the Nadir, the lowest point in
the horoscope: this is where the Sun is at midnight. Since this
point is below the horizon, it is also called lower or midnight
culmination.

The four cardinal points in the house system

As mentioned before, the Ascendant and Descendant, Medium Coeli
and Imum Coeli constitute the four cardinal points of all house
systems in the horoscope. These are generally undisputed when calculating
the houses. There are merely systems which do not use one of the
axes, for example, the ecliptical system which starts from the
Ascendant and neglects the meridian axis.

However, generally we can say that these four points are defined
in the same way in each house system, whether they are used or
not: they are intersection points between the great circles
of the horizon and the meridian with the ecliptic.

The definition of the intermediate houses

The task is now to divide the space between the four intersection
points into three parts. This is best done in a way that makes
sense astrologically.

We could simply divide the
distance between the cardinal points in the horoscope, e.g. between
Ascendant and Midheaven,
into three equal parts. This was suggested by Porphyrius.
But this method does not bear any relation to the visible sky,
it is
simply a mathematical
operation. Consequently, we need to find an organising level
which is manifest in the sky.

The ecliptical systems, for example, use the ecliptic as an organising
level for the house division, others use the vertical great circle.
But in order to define the intermediate houses, astrologers have
also used other reference levels. The two most important ones,
beside the great circles discussed above, are:

the celestial equator and

the diurnal arcs of the celestial bodies.

The celestial equator

Illustration 3: Celestial equator and ecliptic.

This is another important great circle in the celestial sphere
and corresponds to the projection of the Earth's equator onto the
sky. The intersections between the celestial equator and the ecliptic
mark two particularly important astrological points: 0° Aries
and 0° Libra. They mark the beginning of the respective zodiac
sign and at the same time signify the beginning of spring and autumn.
The Aries point has a special meaning: it marks the beginning of
the astrological zodiac and therefore of the astrological year.
The Sun reaches this point at the time of the vernal equinox
(around 21st March): day and night have exactly the same duration,
and
from now on, the days are getting longer than the nights - the
victory of light over darkness (Translator's note: in the Northern
hemisphere! - In the Southern hemisphere, the days are now getting
shorter). Directly
opposite the Aries point, we reach the autumnal equinox - the Sun
enters the sign of Libra around 23rd September (cf. illustration
3).

Two other points are also marked by special positions of the Sun:
the points of Cancer and Capricorn. The Cancer point marks the
position in the zodiac, at which the Sun reaches its highest possible
position above the horizon in the course of a year: The summer
solistice (around 21st June). This is the longest day and the shortest
night in the year (Translator's note: in the Northern hemisphere
- the shortest day and longest night in the Southern hemisphere).
From now on, the Sun turns
to move
downward
again, because the arc it describes in the sky between dawn
and dusk every day becomes smaller and smaller, until it reaches
its lowest point at 0° Capricorn, the winter solistice (around
22nd December). From there, it turns to move upwards again.

In relation to the celestial equator, this means that the Aries
and Libra points constitute the intersection points of this great
circle with the ecliptic, whereas the Cancer and Capricorn points
are the points with the greatest distance between ecliptic and
celestial equator. This distance is also called 'decliniation',
which leads to the statement that at the time of the two
solistices, the Sun has the greatest declination from the celestial
equator. The maximum distance between ecliptic and celestial equator
is fix
at around
23.5° and is called "ecliptical obliquity".

We could also say that the celestial equator marks four stations
on the ecliptic of the Sun's annual orbit around the Earth.

One example for practical use of the celestial equator for
the construction of a house system is this:

Regiomontanus used this great circle by dividing it into twelve
equal parts. First of all, he had to project the four fixed cardinal
points onto the celestial equator, so that their representations
on the great circle would result in four commensurate quadrants:
the Ascendant was consequently represented by the East point which
marks an intersection between horizon and celestial equator, and
the Midheaven was represented by the intersection of the celestial
equator and the upper meridian (i.e. that part of the meridian
which is located above the horizon).

These divisions were then divided into three so that there were
twelve equal divisions on the celestial equator. These had to be
projected onto the ecliptic to make them astrologically visible.

For this purpose, Regiomontaus connected the division points of
the twelve sections on the celestial equator with the celestial
North and South points of the celestial horizon
(which is a great circle parallel to the apparent local horizon).
In this manner, he created twelve additional
great circle arcs which all originate in the North and South points
of the celestial horizon and intersect the celestial equator.
(This is illustrated by the image of an orange with its slices).

Finally, each of these twelve arcs intersects the ecliptic at some point
- and these twelve ecliptic points make up the house cusps of the
horoscope according to Regiomontanus.

The Diurnal Arcs

The diurnal arcs are another important means for constructing
house systems, mainly those which emphasise the time factor,
like
for example Placidus' system.

Diurnal arcs are those semi-circles which are described by
a celestial body moving above the horizon from its ascending point
in the East to its descending point in the West. Each point in
the sky marks such a diurnal arc. The arc resulting from the
movement of the descending point to the next ascending point
is called nocturnal arc. Put together, diurnal
and nocturnal arcs result in a full circle.

In astrology, only those points are important which are projected
onto the ecliptic, usually the planets, Sun and Moon. Astrologically
speaking, these are always located on some point of the ecliptic
- and this ecliptical point also has a diurnal or nocturnal arc.
This means that all points on the ecliptic ascend and descend at
some stage.

Illustration 4: The construction of houses according to Placidus
based on diurnal arcs.

Placidus used these arcs to construct his house system by
saying that a particular point on the ecliptic which has covered
half the distance above the horizon has reached the midday (upper)
culmination, which is, astrologically speaking, exactly at the
point of the Midheaven. This means that this point on the ecliptic
corresponds to the Midheaven as one of the four cardinal points.
And the point on the ecliptic which begins its diurnal arc in the
East, corresponds to the Ascendant. In order to define the intermediate
houses, he concluded that the point which has covered one sixth
of its daily path from East to West, corresponds to the cusp of
the twelfth house, and the point on the ecliptic which has covered
two sixths marks the cusp of the eleventh house, etc. (cf. illustration
4).

If this system is complemented by the nocturnal arcs, the house
system according to Placidus is deduced from the daily movement
of the ecliptic reflected by the diurnal arcs.

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